The present invention relates to an optical dispersion compensator or optical waveguide device which can be applied to an optical transmission system to remarkably increase a transmission distance and capacity thereof.
An optical fiber has a dispersion of refractive index and accordingly when an optical signal is transmitted through the optical fiber, a waveform thereof is deteriorated. Deterioration of the optical signal due to the dispersion of the fiber is a large factor for limiting the transmission distance and capacity of the optical transmission system. Even though the dynamic wavelength shift of an optical source itself can be suppressed by use of a single mode semiconductor laser, or adoption of an external modulation system, spread (.DELTA..lambda..sub.0) of wavelength of the optical signal due to a side-band occurring in high-speed intensity modulation of light wave can not be reduced by adoption of any modulation system. Since the spread .DELTA..lambda..sub.0 is proportional to a bit rate of the optical signal, the transmission distance in a next-generation ultrahigh-speed optical transmission system exceeding Gbit/sec is greatly limited by the unavoidable spread of the wavelength.
If an optical element having dispersion opposite to the dispersion in the optical fiber, that is, an optical dispersion compensator is inserted in the optical transmission path to cancel the dispersion in the deteriorated optical fiber, the optical signal waveform can be restored completely and a long-distance ultrahigh-speed optical transmission exceeding the limited distance can be attained. Methods of using an optical waveguide with a grating to realize a dispersion compensator are disclosed in, for example, Japanese Patent Documents JP-A-55-161201, JP-A-56-1001, JP-A-57-40207 and JP-A-57-66403. However, since these methods are intended to suppress a very large deterioration of waveform caused by multi-mode oscillation of a semiconductor laser, the dispersion compensation is merely performed discretely for mode waveforms and the dispersion compensation can not be performed continuously over the whole wavelength width constituting the optical pulse. A method capable of attaining the continuous dispersion compensation is disclosed in Japanese Patent Documents JP-A-57-129036. This method employs a grating in which pitch is gradually reduced, that is, a chirped grating to be able to perform the continuous dispersion compensation theoretically.
In the dispersion compensator described in the abovementioned Japanese Patent Documents JP-A-57-129036, however, a wavelength area in which dispersion compensation can be performed, the intensity of dispersion compensation and the like are uniquely determined by a pitch of the grating, a structure of an optical waveguide and the like and there is no adjustment measure therefor. The oscillation wavelength of a semiconductor laser used in an actual optical transmission system is scattered over a wide range of several nanometers due to the element dependency, the temperature variation, the secular change and the like. This is ten or more times larger than the spread of a wavelength of the optical signal, that is, a wavelength width requiring the dispersion compensation. If dispersion compensation covering several nanometers is performed, the length of the compensator is ten or more times larger than a conventional device, so that it is difficult to manufacture and an optical loss of the dispersion compensator is increased. Further, the necessary magnitude of dispersion compensation is greatly dependent on the length of an optical fiber and scattered dispersion of the optical fiber. Accordingly, in order to actually adapt the dispersion compensator to the transmission system, it is indispensable to be able to adjust the wavelength and the magnitude of dispersion compensation to some extent after it is manufactured.